Steam generating installations



[56] References Cited UNITED STATES PATENTS 1,709,997 4/1929 Noack....................... 1,835,610 12/1931 United 7 States Patent [72] Inventors Eric Maurice Woolley Saltburu; George Wright, Durham; Roy Crisp, l-lartburn, Stockton-on-Tees; Robert Dennis Atkinson, Mlddlesborough, England 768,390

[21] Appl. No. [22] Filed Page Wood..... 2,614,543 10/1952 Hood.......... 3,164,134 3,234,920

Oct. 17, 1968 Patented 1970 2,065,782 12/1936 [73] Assignee Head Wrightson and Company Limited Stockton-on-T Teesside. England 1 /1965 Kochey, a corporation of the United Kingdom 2/1966 K m etmull r er a1. Oct. 20, 1967, Dec. 20,1967 Great Britain [32] Priority Primary Examiner-Kenneth W. Sprague Nos 47881/67 and 57772/67 Attorney-Baldwin, Wight, & Brown ABSTRACT: A method and installation for i in which, immediately prior to a firin generating steam g cycle, steam is injected into the circuit by means of assistant circulators which steam is effective to raise the temperature in the circuit and promote more vigorous circulation so that, when the firing cycle is commenced, the parts of the circuit are already heated and thermal stresses therein are therefore reduced.

mm u M my 0 m n m1 A m L m L n m ms h- N m Hm m w m Ra r ND m E m Gen m, m L AhC m SHU M H H U PATENTED Bin 8 i970 3545410 SHEET 1 OF 4 PATENTED nEc m SHEET 3 [IF 4 SHEET 0F 4 5 53410 PATENTED DEC 8 I973 STEAM GENERATING INSTALLATIONS This invention relates to steam generating installations and is concerned with installations of the type, hereinafter referred to as of the type described," in which heat, for example waste heat from a steel-making process, is supplied intermittently to a stack incorporating evaporatively cooled surfaces, the steam being passed to thesteam drum of the steam generating circuit of the installation.

Known installations of the type described generally operate either on a natural circulation system or on a pumped circulation system, both. of which have their advantages and disadvantages. i As regards natural circulation, for steam pressures up to around 2,500 p.s.i.g, there is sufficient density difference between steam and .water to design an evaporativelycooled loop to operate under naturalcirculation alone. Such a system has the advantage of avoiding the undesirable feature of having any moving machinery, such as pumps, located in the circuit, or small diameter nozzles or orifices which determine and limit the amount of flow to each heated tube as used in a pumped system. It also avoids the extra energy cost involved for running the pumps. The heated surface, in the natural circulation system however, will use larger size pipes than that possible with the pumped system.

Natural circulation has the disadvantage that, when this is the only method of circulation, the water in the circuit will be stagnant at the time that heat is first supplied to the evaporatively cooled surfaces. In conventional boiler plant,,this heat input is brought on gradually at the commencement of an operation. In someboiler plant the heat is subjected to an on/off control system, but the heat fluxes experienced are not abnormally high.

However, in the case of some processes, for example oxygen steel making, the evaporatively cooled surfaces receive a sudden shock heat approximately every hour. This shock heat can involve very high-heat fluxes well above those normally experienced in conventional boiler practice. Although this problem may be partially overcome in the case of suppressed combustion systems where the time period of full combustion 'is considerably reduced, the problem still'exists that sudden high heat fluxes resulting inthesudden evolution of steam within initially stagnant water lead to undesirable thermal stresses and initial vibration.

A pumped circulation system has the advantage of ensuring that when sudden heat input occurs, this will occur on existing circulation within the cooling circuit. Because pumped circu lation is used, a higher friction resistance can be permitted in the circuit, and this permits the use of smaller bore tubing, with a resultant saving in weight. In the pumped system, as already mentioned, the amount waterflow apportioned to each heated tube is governed by the sizing of an orifice. The heat flow is calculated, and the waterflow matched, so that an acceptable calculated ratio of steam/water will be present at the outlet from the heated tube. For those cases where the water entering the tube is well below the boiling temperature, the orifice additionally ensures stability of flow and correct division of flow between the several tubes connected in parallel.

Whereas the pumped circulation system can save weight by using smaller bore tubes, this also has the disadvantage of reducing the thermal reservoir capacity of the water contained in the tube. Should the heat input in practice exceed that calculated, it is easier .to obtain a dry out condition in these smaller bore tubes. Such a case can arise more easily in than other adjacent tubes, e.g. through particular flame t impingement, then the relative flow through these tubes under natural circulation is mainly self-adjusting, automatically matching circulation to heat input, by the very nature of its design.

It is among the objects of the presentinvention to provide a method of operating an installation, and an installation for carrying out the method, which incorporate a combination of The method further comprises terminating the steam supply when the main heat input hasv become sufficiently established to promote natural circulation.

Thus, it will be appreciated that the method initially calls for operation on the pumped circulation principle and thereafter operates on the natural circulation principle.

The method according to the invention also comprises the step of storing the said steam which is injected into the circuit in a accumulator which is arranged to operate in parallel with the site steam main.

According to the invention furthermore, there is provided an installation of the type described which comprises one or more devices arranged in the steam circuit between the evaporatively cooled surfaces and the steam drum which devices are arranged to inject a supply of steam into the circuit in order to raise the temperature therein, and to promote circulation, so as to reduce thermal stresses particularly in the evaporatively cooled surfaces.

A further feature of the invention is that means are provided for terminating the supply of steam to the said devices when the circuit reaches a predetermined condition.

According to the invention still further, there is provided a steam accumulator which is capable of supplying steam to said devices at any time during the norrnall operation of the cyclic systems receiving intermittent and sudden high-heat fluxes,

such as is experienced in the open combustion period of oxygen steel making processes.

The larger tubes used in natural circulation provide both a larger mass of water within the tube, and a greater freedom for the suddenly formed steam bubbles on the tube wall to be accommodated within the rising steam/water mixture.

Natural circulation has a further advantage over the orifice controlled flow pumped circulation in that it is much more flexible; should, therefore, particular tubes receive more heat heat input equipment of the installation, said accumulator being arranged to operate in parallel with the site steam main.

panying drawings in which:

FIG; 1 is a schematic layout of part of an installation according to the invention, a

FIG. 2 is a schematic layout of a continuation of FIG. I the two parts being joined together at the points marked fX,

FIG. 3 is a section through one form of assistant circulator device,

FIG. 4 is a section, on an enlarged scale, on the line 4-4 of FIG. 3, and

FIGS. 5 to 9 are sections through alternative forms of assistant circulator devices.

Referring to FIGS. 1 and 2 of the drawings, the installation comprises a flue or stack 10, the walls of which are provided with evaporatively cooled surfaces, for example evaporatively cooled tubular panels, the flue or stack 10 being arranged to receive heat intermittently from a source of supply 11 which may, for example, be a steel making converter. The drawing shows the panels of the flue or staclk 10 as being in three separate sections but it will of course be appreciated that one or more panel sections may be provided according to requirements.

Each panel section is connected to an outlet header 12 from which a steam/water mixture passes, via riser pipes 13, to a steam drum 14. Steam is passed from the steam drum via a pipe 15 and feed water is passed into the steam drum 14 through a pipe 16 a steam drum overflow isindicated at 17. Water from the steam drum 14 is passed, via a downcomer l8 and inlet headers 19, to the evaporatively cooled panels.

The mainfeature of the invention is that assistant circulators are arranged to inject steam into each of the riser pipes 13, the steam being passed through supply pipes 21, each incorporating a nonretum valve 22, via common supply lines 23 and 23a under the control of flow control valves 24 or 24a.The common supply line 23a carries steam from site or from another source such as a suitable package boiler.

Although the invention is primarily concerned with maintaining correct conditions in an established or working installation, it also provides advantages where the installation is to be started up from cold.

The operation of the installation described so far will therefore now be described with reference to starting from cold.

When the plant is completely cold it is first filled up with water from the feed water supply pipe 16. Steam is then introduced via the steam supply lines 23 and 23a, and the assistant circulators 20, which steam will condense in the water in the riser pipes 13. The rate of steam flow into these circulators for warming up is controlled by the flow control valve 24a.During this period the heating steam which is also condensed will cause a change in density of the water in the riser pipes 13 and this will establish thermal circulation in the system. As the steam continues to be injected, the temperature of the water will be increased until first reaches boiling point at atmospheric pressure; the air cock of the boiler steam drum 14 is then shut. The assistant circulators 20 will then continue to feed steam into the circulation loop and increase the steam pressure with a corresponding rise in the temperature of the water now at saturation conditions are reached (i.e. boiling condition).

When the pressure in the steam drum 14 reaches the minimum accepted operating pressure, the flow control valve 24a will be closed, and the further flow control valve 24 preferably of a larger capacity can be opened. This valve is opened just before the system is due to receive heat from the heat supply source 11. As the water is now at saturation condition the steam bubbles coming from the assistant circulators 20 will, therefore, cause a steam/water mixture to exist in the riser pipes 13, thereby setting up a more vigorous circulation in the system due to the density differential existing between the water in the downcomer 18 and the steam/water mixture in the riser pipes 13. Steam will be bled from the drum 14 as required, in order to maintain sufficient driving pressure for the circulators.

Thus, it will be appreciated that there will already exist, in the cooling panels, a flow of water when the heat supply source 11 is initiated. This flow will provide a turbulent, scrubbing effect on the surfaces of the cooling panels and will therefore minimise thermal shock, and the vibrations which would otherwise result from the sudden ebullition of steam bubbles in a stagnant fluid.

Once the heat input from source 11 has become established, the pressure in the steam drum 14 can be permitted to rise at a controlled rate. In the case where the steam supply pressure to the assistant circulators 20 is below the system steam pressure, as the system pressure rises, the valves 22 will close shutting off steam to the assistant circulators 20. From this point of time forward the system operates completely as a natural circulation system, while the heat input continues.

At the end of a firing operation, when the heat input is shut off, the system will start to cool down and this will result in a drop of steam drum pressure. If needed the rate of steam drum pressure drop can be controlled, the system flashing off steam as the pressure is dropping thus maintaining the water in the circuit at the saturation temperature corresponding to the pressure then existing. This will mean that a turbulent state will exist in the form of steam/water mixture throughout the circuit.

Eventually the pressure within the circuit will fall to that which is lower than the steam supply feeding the assistant circulators 20 and the warming through and circulation promotion steam can again be admitted by the use of flow control valves 24 and 24a. The cycle described above then recommences.

The installation may further include a main steam storage accumulator 25 which is provided, in generally known manner, to receive steam in excess of site requirements and is, for this purpose, connected via lines 26 and27 in parallel with the site steam main l5. Condensers 28 are also arranged via line 29 in the site steam and accumulator circuit and provide a feed water supply which passes to the steam drum 14 via the pipe 16 using a feed pump.

In accordance with a further feature of the invention, steam for supplying the assistant circulators 20 is stored in an assistant accumulator 30. For this purpose, the supply pipes 21 are in communication with the supply line 23 which is connected to the accumulator 30 by a line 31. The accumulator 30 is further connected, via steam lines 32, to the site steam main 15.

The installation incorporating the accumulators 25 and 30 operates as follows:

At the commencement of a campaign, when the cooling circuit comprising items 14, 18, 10, 20 and 13 has been filled up with cold water, the circuit is at atmospheric pressure. Steam is then admitted to the circuit via the steam lines 23 and 23a, and the assistant circulators 20, at first raising the temperature of the circuit to boiling point at atmospheric pressure. Continuation of the steam supply via the circulators 20 will gradually raise the pressure and temperature of the water and the temperature of the metallic members carrying the water through the circuit, until it reaches a pressure approaching that of the supply source which, in this case, is steam from the site which is passed thereto via line 23a. At this stage, the rising steam bubbles from the assistant circulators 20 in the riser pipes 13 will cause circulation of the water in the circuit of sufficient rate to permit the initiation of the am main heat source 11. When the main heat source commences operation, this will generate more steam in the circuit causing a pressure rise in the circuit above that of the site steam main supply 15, and thus shut nonretum valves 22. As the heat input continues, the pressure will be permitted to rise in the circuit at a controlled rate until the desired maximum has been reached. This pressure rise also causes the circuit itself to act as a heat or steam storage unit.

The assistant accumulator 30 is connected in parallel with the now heated circuit via the lines 2l, 23, 31 and 32 the line 32 including a nonretum valve 33. The size of the accumulator 30 is such that it will rise in pressure at the same rate as the heated circuit and store, for a relatively small pressure rise, all the steam needed for the operation of the assistant circulators 20.

Once the campaign has started and the heat circuit has received its first heat input, then the operation of the installation is automatic so that each time the cycle recommences, steam is supplied to the assistant circulators 20 from the assistant accumulator 30.

The assistant circulator accumulator 30 is sized so that it can amply provide circulation of the water at the beginning, and just prior to, the commencement of the heat input, and continue the operation so there exists a reasonable overlap period of the operation of the assistant circulators 20, with the commencement of heat input.

Should the heat input ever be interrupted then the steam drum pressure will be dropped automatically to a pressure below that of the pressure of the assistant accumulator 30 so 'that the required steam flow can be obtained through the circulators 20.

Under normal operative conditions a valve 34 will be opened by a signal which, with a slight time lag, also initiates the heat input source 11, such that it always follows behind, in time, the commencement of the flow of steam through the assistant circulators 20. Control of the valve 34 is such that it maintains a relatively constant flow of steam into the circuit.

Thus, it will be appreciated that, at the beginning of each cycle, a signalstarts the operation of valve 34 letting steam enter the assistant circulators and thus establishing a water circulation in the circuit 14, 18, 10, 20 and 13. The heat input then commences and the steam generated will cause a rise in system pressure. The control system is such that the rate of system pressure rise is controlled to a rate acceptable to the pressure parts. The assistant accumulator pressure will rise at the same rate.

At the end of the heat input period, the steam pressure in the circuit is allowed to fall at a controlled rate thereby performing two functions in that, firstly, it releases steam stored in the circuit to thesite as it drops in pressure, and secondly, as the pressure drops, it causes the water to be maintained at boiling temperature throughout the circuit, thus avoiding the undesirable existence of uneven temperature distribution as i well as excessive subcooled water throughoutthe circuit, i.e.

below its boiling temperature.

Thus, it will be understood that, by providing the steam accumulator for the assistant circulators 20 in accordance with the invention, and by suitably controlling the steam circuit, there is provided an installation wherein assisted circulation can be provided at any time during the cycle of operation, including those occasions when the main heat input source is interrupted. e t

It will further be appreciated that the steam accumulation in the circuitry is also such that it enables steam to be distributed to the site over a wide cycle of operation.

Thus, it will be appreciated that the invention provides the following advantages: i

I. It combines the desirable advantages of both the natural circulation and the assistant circulation systems and provides positive circulation for a short period just prior to the sudden heat input, without the use of expensive pumps, associated orifices and small bore piping.

2. The evaporatively cooled walls can be of the more robust construction associated with the larger bore tubes of a natural circulationsystem, coupled with the related advantages of an increased thermal reservoir capacity, while avoiding the disadvantages of the pumped system.

. It avoids the excessive cooling which unnecessarily exists with the pumped circulation system which latter gives higher heat transfer rates from thewalls to atmosphere during the off blowperiod; unless the pumped system incorporates a complicated and expensive shut off control system with isolation valves. i Y t 4. It shows a cost saving over the pumped system on both capital and running costs.

5. When using the waste heat from larger sized converters, with their high-heat loads, a positive circulation in the cooling circuits at the beginning of each blow becomes necessary. The invention provides this circulation in a simple and advantageous way and in a manner which is effective to reduce thermal stress and vibration when starting up. i

It will be notedthat numerous valves and other control means are shown in FIGS. 1' and 2 but have not been specifically described; The reason for this is in order to simplify the description since such control means, although of importance to the practical operation of the installation, are well known and are not necessary to give a clear understanding of the the assistant circulators 20 are used to inject steam into the.

circuit for two separate purposes as follows:

I. For warming up the system from cold; and 2. After the completion of the warming operation, for setting up a circulation in the circuit by establishing a steam/water mixture in the risers 13.

During the warming up period, all the steam injected into the circuit is condensed. During the second period, i.e. the period of steam injection for establishing circulation via a steam/water mixture, some condensation of the injected steam may occur. The proportion of condensation of steam thus in jected will depend on its vertical location below the steam drum l4, and the resultant static head. The static head, or pressure, will result in the water at the point of injection being at a temperature below boiling and therefore will be, able to absorb heat from the steamwith the result that some condensation occurs. As the steam/water mixture ascends in the risers 13, the portion of the condensed steam will flash back again into steam.

Therefore, in order to avoid vibration and noise resulting from any steam condensation, and steam slugging from the ascendingsteam bubbles in the risers 13, special attention must be given to the nozzle design and its location. Thus, when the steam bubbles are large, and are suddenly condensed, this sets up bothvibration and noise in the riser tubes 13. Furthermore, with an incorrectly designed nozzle, it is possible that condensation may occur so quickly that the liquid is drawn back into the, nozzle bore itself, from which it suddenly ejected. This can setup oscillations resulting in further unacceptable severe vibration and noise.

Furthermore, when steam bubbles are rising in a tube, the larger size bubbles will grow in size by absorbing the smaller bubbles in their passage and this could result in the bubble size being such that a slug of steam exists across the tube diameter. Thus, the assistant circulators should be designed so that the injection nozzles thereof avoid an injection location which can cause steamslugging to occurat the point of entry of the induced waterflow, and should comply with the following? 1. Steam injection should occur insulch mannerthat the ini duced waterflow is concentric and in parallel flow, in a straight vertical part of the riser tube, with the steam injection entry, preferably at the center of the riser tube, in

i order to minimise bubble coalescenceon the wall of the riser tube.

2. The steam nozzle injection holes should be of small diameter and meet the requirements I concerning clearance as hereinafter described. i

3. The core of the injection nozzle should be packed with steel wool, or the equivalent, as also should the space between the nozzle and the nozzle branch entry pipe, where applicable.

Referring to FIGS. 3 and 4 of the drawings, an assistant circulator comprises an arc'uate tube section 35 which is coupled to, or forms part of, each riser tube 1.3, the tube section 35 being formed with a branch entry pipe 36which is disposed tangentially with respect thereto. I

A steam injection tube 37 having an injection nozzle 38 at one end thereof, is arranged coaxially within the branch pipe 36 to extend into thetube section 35 sothat the nozzle 38 is disposed concentrically within a straight portion of the tube section 35. At the junction of the tube section 35 and the branch pipe 36 there is provided an injection tube guide plate 39, the branch pipe being packed with stainless steel wool or the equivalent indicated at 40. i

The injection tube 37 is adapted to be connected to the steam line 21 and is also packed with stainless steel wool or the equivalent indicated at 41. t

The injection nozzle 38 is tapered and is drilled with smal diameter holes 42 which are inclined upwardly so as to facilitate the rising flow of small bubbles of steam in the direction of the arrows 43. The vertical pitch p of the holes42 is such that there exists, in plain view, a clearance c from one set of holes to the next adjacent set.

. Thenozzle 38 is formed with four spaced longitudinally disposed stifi'ener plates and guide vanes 44, and is closed at its forward end by a cap 45.

i The steam injection tube is located, at its forward end, concentrically within the tube section 35 by means of four adjustable steady rods 46 which engage the edges of the stiffener plates 44 which, for this purpose, are provided with reinforcing blocks 47.

For the purpose of centralizing the injection tube 37, a closure plate 48 for the branch pipe 36, which plate is fixed to the tube 37, is adapted to be located by two dowel pins 49 provided at different centers on a flange 50 fixed to the pipe 36.

FIG. shows another form of assistant circulator in which steam is passed into the tube section 35 via a stuffing box 51 packed with wire wool or the like 52, the forward end of the stuffing box 51 being provided with an apertured plate 53 arranged so that the steam pa'sses into the waterflow in the same direction as the movement of said flow.

FIG. 6 shows an arrangement similar to FIG. 3 but of simplified form.

FIGS. 7, 8 and 9 show further forms of circulators where the branch pipe is dispensed with and steam in injected into the tube section 35 through a transversely disposed pipe 54 which is adapted to be coupled to the line 21. In FIG. 7, the pipe 54 is connected to a crossflow nozzle 55 which extends across the diameter of the tube section 35 and is formed with apertures 56 through which steam is passed into the waterflow. In FIG. 8, the pipe 54 is connected to a nozzle ring 57 arranged within the tube section 35 and having apertures 58 through which the steam is passed'into the waterflow. In FIG. 9, the pipe is connected to an annular venturi type nozzle 59 arranged within the tube section 35 and having apertures 60 through which the steam is passed into the waterflow.

We claim:

1. A steam generating installation comprising a stack adapted to be intermittently supplied with an external heated medium, said stack including evaporation means including evaporatively cooled surfaces, a steam drum located above said stack, first conduit means for conducting cooling water from said steam drum to said evaporation means, second conduit means for conducting a steamwater mixture from said evaporation means to said steam drum, and means for injecting steam into said second conduit means at a position below said steam drum sufficient to establish a density differential between the cooling water in said first conduit means and the steam-water mixture in said second conduit means to achieve circulation in the system from said evaporation means through said second conduit means to said steam drum prior to the supplying of the heated medium to the stack thereby reducing thermal stresses in the system and particularly the evaporation means thereof.

2. The steam generating installation as defined in claim 1, including means for retarding the flow of steam through said steam injecting means whereby the steam, when injected into said steam-water mixture, is reduced to a mass of small diameter bubbles.

3. The steam generating installation as defined in claim 1 including means for retarding the flow of steam through said steam injection means whereby the steam, when injected into said steam-water mixture, is reduced to a ms mass of small diameter bubbles, and said retarding means is intersticesdefining means housed within said steam injection means.

4. The steam generating installation as defined in claim 1 including means for intermittently supplying the external heated medium to the stack.

5. The steam generating installation as defined in claim 1 wherein the circulation is established solely by the density differential in the absence of means other than the injection of steam into said second conduit means.

6. The steam generating installation as defined in claim 1 wherein means are provided for terminating the supply of steam to said steam injecting means when the system reaches a predetermined condition of generally stable circulation.

7. The steam generating installation as defined in claim 1 including an onsite source of steam, and means coupling said onsite source of steam to said steam injection means.

8. The steam generating installation as defined in claim 1 including a small package boiler for supplying steam to said steam injecting means.

9. The steam generating installation as defined in claim 1 including steam accumulator means for supplying steam to said steam injecting means.

10. The steam generating installation as defined in claim 1 wherein said steam injection means comprises a tube section connected'to said second conduit means, a branch pipe extending generally tangentially with respect to said tube section, a steam injection tube extending through said branch pipe and into said tube section, said steam injection tube having an apertured nozzle disposed in a straight portion of said tube section, and at least one of said branch pipe and said in jection tube being packed in with means defining interstices for retarding the passage of steam therethrough.

11. The steam generating installation as defined in claim 10 wherein said injection tube nozzle is provided with axially extending radially spaced stiffening plates and guide vanes.

12. The steam generating installation as defined in claim 10 including adjustable steady rods for positioning the steam injection tube generally centrally of the tube section.

13. The steam generating installation as defined in claim 1 in which said steam injecting means includes a tube section connected to said second conduit means, a tangentially disposed stuffing box attached to said tube section for receiving steam from a steam source, an apertured plate provided at a junction between the stuffing box and the tube section, the stufiing box housing means defining interstices to retard the passage of steam therethrough, and said apertured plate being constructed and arranged so that steam is injected into a straight portion of the tube section generally parallel to the waterflow therethrough.

14. The steam generating installation as defined in claim 1 wherein said steam injecting means comprises a tube section connected to said second conduit means, and an injection nozzle adapted for connection to a source of steam arranged transversely within the tube section so as to inject steam into the latter in a direction parallel to the waterflow therethrough.

15. The steam generating installation as defined in claim 1 wherein said steam injecting means comprises a tube section connected to said second conduit means, a crossflow tube extending diametrically across said tube section and being adapted for connection to a source of steam, and said crossflow tube housing means defining interstices for retarding the flow of steam therethrough, and a plurality of apertures in said crossflow tube for the passage of steam outwardly therefrom.

16. The steam generating installation as defined in claim 1 wherein said steam injecting means comprises a tube section connected to said second conduit means, a tubular ring extending about the inner periphery of said tube section, said ring being provided with a plurality of apertures and being adapted for connection to a source of steam, and said ring housing means defining interstices for retarding the flow of steam therethrough.

17. The steam generating installation as defined in claim 1 wherein said steam injecting means comprises a tube section connected to said second conduit means, an annular hollow venturi tube extending around the inner periphery of said tube section, said annular venturi tube including an innermost conical surface provided with a plurality of apertures, and said venturi tube housing means defining interstices for retarding the flow of steam therefrom.

18. A method of operating a steam generating installation which includes a stack having evaporation means including evaporatively cooled surfaces, a steam drum located above the stack, first conduit means for conducting cooled water from the steam drum to the evaporation means and second conduit means for conducting a steam-water mixture from the evaporation means to the steam drum comprising the steps of injecting steam in the form of small bubbles into the second conduit means at a position below the steam drum to establish a density differential between the cooling water in the first conduit means and the steam-water mixture in the second conduit means to achieve circulation in the system from the evaporator means through the second conduit means to the r 9 l t s r 10 steam tank and performing the latter step prior to supply sup retarding the flow of steam and generating the steam bubbles plying a heated medium to the'stacl c. A therefrom by passing the steam through interstices prior to the 19. The method as defined' in claim 18 including the stepof injection thereof into the step mwater mixture. 

